PHT doesn't require Transcoding. Its a plex client so plays everything you throw at it
Well I gave up and put Windows 10 on the NUC and Plex Home Theater runs perfectly. It crashed once when fast forwarding, but I think that was a one off. Once I mucked about with the audio settings I got DTS HD and MA pass through to the amp. Very impressed with the UI!
Also put it on a Pi B in the kitchen, again works well but wireless buffering is letting it down. I may get a powerline to it somehow.
So far I'm happy I made the switch.